Understanding Plate Girders and Web Stiffeners

Plate girders are commonly used in structural engineering for large spans where rolled steel sections are not sufficient. They are built up by welding or bolting together steel plates to form an I or H section. A typical plate girder consists of flanges (top and bottom plates) to resist bending moment and a web (a vertical plate) to resist shear force.

The web of a plate girder is often thin compared to its height. This slender web is susceptible to buckling under shear and bending stresses. To increase the buckling strength of the web, stiffeners are added.

Types of Web Stiffeners

Stiffeners are steel plates or sections attached to the web. They are primarily of two types:

• Vertical or Intermediate Stiffeners: These are placed vertically, usually at intervals along the length of the girder, between the support and the load points. Their main function is to improve the shear buckling strength of the web.
• Horizontal or Longitudinal Stiffeners: These are placed horizontally along the length of the girder. They help in increasing the bending buckling strength of the web, especially under concentrated loads or in areas of high bending stress.

When both vertical and horizontal stiffeners are used, or when only vertical stiffeners are used, the web is divided into rectangular areas called panels.

Limit on Panel Dimensions for Vertical Stiffened Web

For a web stiffened with vertical stiffeners, the dimensions of the rectangular panels formed by the stiffeners and the flanges are crucial for preventing buckling. Design codes specify limits on these dimensions.

The question asks about the limit on the "lesser clear dimension of the panel" for a vertically stiffened web. The clear dimensions of a panel are the distances between the stiffeners (clear horizontal distance) and the distance between the flanges (clear vertical height of the web).

For a panel in a vertically stiffened web, the design rules stipulate that the lesser of these two clear dimensions should not exceed a certain multiple of the web thickness.

Let:

• \(d_c\) = clear height of the web between flanges
• \(c\) = clear horizontal distance between vertical stiffeners
• \(t_w\) = thickness of the web plate

The dimensions of a web panel are \(d_c\) and \(c\). The lesser clear dimension is \(min(d_c, c)\).

According to design codes for steel structures (like IS 800), for a vertically stiffened web panel, the lesser clear dimension of the panel should not exceed \(180t_w\).

This means \(min(d_c, c) \le 180t_w\).

This limit helps ensure that the web panel is not excessively slender, thereby preventing premature shear buckling.

Justification of the Limit

The limit of \(180t\) is related to the slenderness ratio of the web panel, which is a key factor in determining its buckling capacity. A higher slenderness ratio (larger dimensions relative to thickness) leads to a lower buckling resistance. By limiting the dimensions, especially the lesser one relative to the thickness, the code ensures a minimum level of buckling strength for the web panel under the applied loads.

Therefore, the lesser clear dimension of the panel in a vertically stiffened web of a plate girder should not exceed \(180t\).

Revision Table: Plate Girder Web Stiffeners

Additional Information: Plate Girder Design Concepts

Here are some related concepts relevant to plate girder design:

• Aspect Ratio: The ratio of the clear depth of the web to the clear spacing of vertical stiffeners (\(d_c/c\)) or vice versa is called the aspect ratio. This ratio also influences web buckling and is often limited in design codes. For vertically stiffened webs, \(d_c/c\) is typically limited to values like 1.5 or 2 depending on the stress levels and code provisions.
• Bearing Stiffeners: These are heavy stiffeners provided at supports and points of concentrated load application. Their primary function is to prevent web crippling and web buckling under concentrated compressive forces.
• Slenderness Ratio of Web: The overall slenderness of the web is often characterized by the ratio of its clear depth to thickness (\(d_c/t_w\)). This ratio is a primary factor in deciding whether stiffeners are required and what type (vertical, horizontal, or both) should be used.
• Buckling Modes: Web can buckle in different ways: shear buckling (diagonal waves), bending buckling (waves parallel to flanges), or local buckling under concentrated loads (crippling). Stiffeners are designed to enhance resistance to these modes.

Understanding these concepts is essential for the design of stable and efficient plate girders.